The yellow ink used for the orange 6¢ contained ferric oxyhydroxide (the chemical compound of Iron), and as everyone knows iron rusts. Normally the oxidisation occurs in the ink drying process. However as the Bureau had, supposedly, a tight control on this process, it was doubtful that this was the cause of the ink oxidising. Another potential source of oxidisation is a reaction to acid. And here is where the mailbags come into focus. The mailbags at the time frequently got wet, they were made out of canvas and without some kind of preventive action they would mould. And mouldy mailbags is not what you would want your mail to be carried in. The mailbags were treated with a special chemical (probably ethylene oxide), one that prevented mould and also prevented the breakdown of the canvas material. The chemical would be allowed to absorb into the bag, and after spraying, would be available for use 24 hours later. But, if the bag was used before then, the chemical would not have absorbed fully and would have reacted to the iron compounds used in the ink on the stamp.

#558 with oxidation at the top and left of the design

#558 First Day Cover, November 10th 1922

A pane of 100 stamps of #558, there were 4 panes to a sheet of 400.